behavior of combine_words when vector length is 2 and the and option is empty

[eitsupi]

Thank you for your continued development of great packages.

I was confused when the vector length given to combine_words was 2.

> knitr::combine_words(c("a", "b", "c"), before = "`", and = "")
`a`, `b`, `c`
> knitr::combine_words(c("a", "b"), before = "`", and = "")
`a``b`

The outputs I wanted were `a`, `b`, `c` and `a`, `b`.
Currently, to get the output of like that, we need to set and to the same content as sep, and also set oxford_comma to FALSE.

> knitr::combine_words(c("a", "b", "c"), before = "`", and = ", ", oxford_comma = FALSE)
`a`, `b`, `c`
> knitr::combine_words(c("a", "b"), before = "`", and = ", ", oxford_comma = FALSE)
`a`, `b`

How about changing to separate words with sep when and is empty even if the length is 2?

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> xfun::session_info('knitr')
R version 4.1.1 (2021-08-10)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 20.04.3 LTS

Locale:
  LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
  LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
  LC_MONETARY=en_US.UTF-8    LC_MESSAGES=C             
  LC_PAPER=en_US.UTF-8       LC_NAME=C                 
  LC_ADDRESS=C               LC_TELEPHONE=C            
  LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

Package version:
  evaluate_0.14   glue_1.4.2      graphics_4.1.1  grDevices_4.1.1
  highr_0.9       knitr_1.34      magrittr_2.0.1  methods_4.1.1  
  stats_4.1.1     stringi_1.7.4   stringr_1.4.0   tools_4.1.1    
  utils_4.1.1     xfun_0.25       yaml_2.2.1 

[yihui]

This sounds like a bug. @cderv Could you take a look and fix it? I guess it shouldn't be hard to fix. Thanks!

[eitsupi]

Thank you for the quick fix!

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